Phosphate in water has been a growing environmental problem as a result of the increased amount of phosphate entering bodies of water from point and nonpoint sources. One specific problem related to waste phosphorus is eutrophication of water, i.e., “the enhancement of phytoplankton growth because of nutrient enrichment” (Pierzynski, et al., 2000). The eutrophication of bodies of water has many negative effects on the aquatic biome and can eventually lead to severe economic, environmental, and human health problems. An increase in the amount of phosphates in a body of water leads to the growth of aquatic weeds and algae (algae bloom). An algae bloom decreases the amount of oxygen in the water and decreases the visibility because of increased surface plant growth, which causes the elimination of bottom-dwelling plants and organisms. The use of the body of water is then discontinued for recreational purposes while cost of maintenance increases. The decreased amount of dissolved oxygen in water is a result of increased amount of microbial growth that feeds off of plant residues. A low level of dissolved oxygen leads to the decrease and elimination of many aquatic species (fish kill). Algae blooms and fish kills then cause odors and insect problems. In addition, the body of water becomes shallow and has limited movement (dead zone).
In some situations, algae blooms can result in human health hazards. Blue-green algae (cyanobacteria) naturally release toxins which in great amounts kill livestock and create a human health hazard. Pfiesteria piscicida is a dinoflaggellate that releases toxins that kills fish and causes lesions. Exposure to Pfiesteria causes symptoms of “headache, blurred vision, sores, reddening of the eyes, memory loss, and cognitive impairment” (Pierzynski et al., 2000).
Struvite (magnesium ammonium phosphate hexahydrate) is a naturally-occurring mineral found in manure and guano, as well in pathologies of urine and the renal tract. It has been a growing problem in wastewater treatment plants because of nucleation in unplanned locations resulting in crystal accumulation in pipes and flow reduction (Rawn et al., 1939). The removal of struvite crystals from pipes is very expensive. Currently, in order to eliminate the possibility of the precipitation of struvite and to reduce the amount of phosphates leaving in wastewater, iron and aluminum salts are intentionally added to precipitate the phosphate in wastewater.
Certain earlier methods of precipitating struvite either induced nucleation from supersaturated solutions with the presence of cellular membranes (Gonzalez-Munoz et al., 1996) or utilized kinetic energy added with high-speed propellers (Ohlinger et al., 1999). Since these methods are not particularly efficient, developing new methods to induce the nucleation of struvite can prove beneficial for controlling nucleation of struvite in wastewater treatment plants and reduction of the amount of phosphate entering bodies of water. Recovered phosphate in the form of struvite may be advantageously used as a slow release fertilizer because of its limited solubility and also merits an “organic fertilizer” designation because of its source.
Prior work with crystallization utilizing membranes has been in regard to the nucleation of calcium carbonate (CaCO3). Monolayer films of stearic acid (CH3(CH2)16CO2H), octadecylamine (CH3(CH2)17NH2), octadecanol (CH3(CH2)17OH), and cholesterol (C27H45OH) have been used under full and partial compression (Mann et al., 1993). Striking results have been obtained with inducing the oriented formation of vaterite, a rare polymorph of CaCO3, in a system that otherwise crystallized calcite. Work with CaCO3 has focused on the structural and stereochemical relationship between the monolayers and controlled nucleation of CaCO3 using monolayers (Mann et al., 1993).
There exists considerable interest in recycling Phosphorus (P) as struvite based on philosophical, environmental, economic and commercial reasons. The European Union goal is to recover and recycle at least 25% of waste phosphorus. Moreover, declining phosphorus content of exploitable virgin ore in combination with the commercial value of struvite at, for example, $320/ton increases the desirability of alternate methods of production of struvite.
Following is a list of various pilot programs in different countries that have existing methods and techniques for removing and recycling phosphorus containing waste. A method used in one pilot program includes using fluidized beds with no addition of chemicals. In this method, however, there is an insufficient quantity of Magnesium (Mg) for proper struvite precipitation. Another method, such as that used in Treviso, Italy, includes using a combination of Mg(OH)2 and NaOH to bring the Mg/P stoichiometric ratio to 1 and increasing the pH so that struvite is precipitated as pellets in fluidized bed reactors.
In Geestermerambacht, Netherlands, the process of phosphorus removal currently requires the initial removal of carbonates from treated liquors at pH 6, followed by addition of Ca(OH)2 to raise the pH to 8–8.5 which causes phosphate precipitation. This method is however, environmentally and economically unattractive.
In a Japanese pilot program, seawater is used as a source of Mg for small Struvite recovery, while in another pilot project, also in Japan, pH of the liquor is adjusted using NaOH. In Sweden, the KEPRO® process is used in a pilot plant for recovery of iron phosphate from sewage. Ferric phosphate, however, produced in the KEPRO® process is not water soluble, and bioavailability of the resulting fertilizer is unclear. In yet another pilot program in the UK, pH is adjusted using Mg(OH)2, which in turn also provided Mg for struvite precipitation.
Occasionally, CO2 stripping is used to raise pH to obtain favorable conditions for struvite precipitation. However, microbial bodies commonly present in the waste sources of phosphorus produced CO2 inevitably and constantly. This method is therefore not particularly effective. Additionally, such stripping does not raise the pH to desirable basic levels that favor struvite precipitation. Yet another method for obtaining struvite precipitation includes dosing the waste phosphorus with MgCl2. Other basic compounds such as NaOH, MgO and Ca(OH)2 have been used to raise the pH. However, these methods increase the salt load, which in turn increases the solubility of sparingly soluble salts contained in the waste, including struvite, which is undesirable for purposes of struvite recovery. Such additions also increase the cost of production. Occasionally, ion exchange resins are used to concentrate reactants such that struvite formation is favored. However, such systems require concentrated brine to strip resin columns, which also lead to reduced efficiencies and economies.
The earliest struvite recovery operation appears to be that of Unitika Ltd (Japan), which has had a “Phosnix” process operating since 1998 in Shimane Prefecture using Mg(OH)2 and NaOH to precipitate struvite. This business has sold its product as a “boutique” fertilizer (Munch et al., undated), presumably as an “organic” or recycled fertilizer sold retail in small packages at prices of $300–800/ton (CEEP, 2001).
Probably the best economic analysis regarding P removal and recovery is that of a pilot plant designed for the Slough (UK) Sewage Treatment Works (Jaffer et al., 2002), which handles 520 kg P in 64,000 m3 sewage per day, from a population of 250,000. Construction of a precipitation reactor was estimated at $27 k. To precipitate 42 to 99 tons of struvite per year would cost $86–$88 k per year. Almost all of the operating expenses were chemical costs; 97% of that cost was NaOH ($94/ton) to raise pH and the rest was MgCl2 ($144/ton) to supplement the magnesium content of the wastewater. Revenues for sale of struvite (at $320/ton) produced were estimated to be $13–32 k, and therefore would cover one-third to one-half of the cost of struvite removal. According to Jaffer et al. (2002), “[c] osts of production have to be offset against the revenue lost through increased pumping costs, lost man hours, expensive pipe replacements, possible excavation work if pipes are located underground and STW downtime due to blockages.” Clearly, reduction in chemical costs, mainly NaOH, would alter the financial balance. A full-scale reactor in Slough was under construction in 2001 (CEEP, 2001).
Additional struvite recovery plants include the Hiagari Sewage Treatment Plant in Kitakyushau City (Nishihara Corp., Ltd, Japan—NaOH), Osaka (Kurita Water Industries—NaOH and MgCl2), and Oxley Creek, Brisbane, Australia (Brisbane—Water and Mg(OH)2). In yet another process, aka, the REM NUT process (Liberti et al. 2001) phosphate and ammonium are accumulated from the effluent on ion exchangers, which are then stripped with brine, forcing struvite precipitation from the concentrated solution. Other related plants include a calcium phosphate recovery reactor (Geestmerambacht, The Netherlands) and iron phosphate recovery pilot plant (Helsingborg, Sweden). [CEEP, 2001].
Additional consideration has been given to struvite formation from manure lagoons, either in the presence of biosolids (which presents formidable problems of separating the struvite from the organic solids) or in filtrates after dewatering, at which point the nature of the chemical problem is similar to that of sewage treatment plants. Removal of P from the manure would better balance the N/P fertility needs of most crops and soils and better rationalize the application of high rates of manure (or biosolid) application for N fertility, without overapplying P.
The need exists, accordingly, for new methods of struvite crystallization such that phosphorus and other minerals may be recycled effectively, efficiently and economically.